At first I would agree with this statement, but then why couldn't they have the actor give directions. "I need you recover that whozzywhatsitz. It's in Rockyweed Cavern. Go north out of town and then follow the road west toward markarth. It will be on the right"
I gues expense of paying someone for the extra dialogue could be a problem, but you could just as easily put, instead of "I'll mark it on your map", "Here, take these directions" and have it added to your journal. That way the directions are there in the quest journal for you. Heck, leave the "I'll mark it on your map" if you want, or don't, and still put the directions in the journal without any voice acting dialogue.
The only problem would be radiant quests that pick a random location.
Well as I see it the problem isn't one of simple resources, but a problem of procedural dialogue. An NPC can be scripted to give directions from one place, but what if you wish to ask a random NPC directions? They can't all have voiced dialogue for every quest, from every location.
By procedural dialogue I mean generated on-the-fly, so you ask your first NPC directions, they have no idea, never heard of it. So you ask another. He tells you to travel X miles down Y road to Z town, and ask again. You go there, and ask another random NPC, they give you more finer directions, travel X miles north-west till you see a tower, then travel north from there. Those are the sorts of directions that can be most easily generated and displayed via text dialogue, but currently cannot be easily generated by voiced dialogue.
And so we have the solution of spamming your map, and providing a magic arrow. But this solution is removing exploration and AI/world interaction, there is generally little reason to talk to NPCs (unless you're told to by your arrow) other than to try & fish for random quests, and it seems to me that world exploration is one golden way to improve this.